1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and more particularly, to a method of manufacturing semiconductor devices capable of preventing variation of the threshold voltage depending on internal diffusion of an ion for adjusting the threshold voltage.
2. Background of the Related Art
Recently, as the degree of integration in the memory devices is increased, the size of the memory cell is abruptly reduced. Therefore, in order to secure the ratio of the wafer to the memory cell, an isolation film using a shallow trench is employed.
In a conventional process of manufacturing the flash memory device, after a well is formed in the semiconductor substrate, a tunnel oxide film and a polysilicon layer to be used as the gate are sequentially formed. The polysilicon layer and the tunnel oxide film are then patterned by means of an isolation mask. Next, the exposed semiconductor substrate is etched, by a given depth, to form a trench of a shallow size. Thereafter, the edge portion of the trench is made rounded by annealing process under hydrogen atmosphere. A nitride film is then formed on the entire structure using Si3N4 gas. After an oxide film is thickly formed on the entire structure so that the trench is buried, a planarization process is implemented to form an isolation film within the trench.
If this conventional method is used, damage of the tunnel oxide film at the top edge of the trench does not occur. However, as a boron (B11) ion implanted in order to control the threshold voltage is experienced by transient enhanced diffusion (TED) in a subsequent annealing process, the concentration of the ion at the substrate surface is increased or reduced and the threshold voltage is thus varied. Due to this, in case of the NAND type flash memory cell in which erase is performed in the FN tunneling mode, the length of the channel is changed to lower electrical characteristics and reliability of the device. Further, as the ion for adjusting the threshold voltage is implanted in the minimum dose, the threshold voltage at the entire regions of the wafer is nit safely secured due to diffusion of the ion.